It is highly desirable for tires to have good wet skid resistance, low rolling resistance, tear strength, and good wear characteristics. It has traditionally been very difficult to improve the wear characteristics of a tire without sacrificing wet skid resistance and traction characteristics. These properties depend, to a great extent, on the dynamic viscoelastic properties of the rubbers utilized in making the tire.
In order to reduce the rolling resistance and to improve the treadwear characteristics of tires, rubbers having a high rebound have traditionally been utilized in making tire tread rubber compounds. On the other hand, in order to increase the wet skid resistance of a tire, rubbers that undergo a large energy loss have generally been utilized in the tread of the tire. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for automobile tire treads.
It is conventionally believed to be desirable for styrene-butadiene rubber that is utilized in tire tread compounds to have a high level of vinyl content (1,2-microstructure). It is also generally desirable for the repeat units which are derived from styrene to be randomly distributed throughout the polymer chains of the rubber. To achieve these objectives, styrene-butadiene rubbers are often synthesized by solution polymerization that is conducted in the presence of one or more modifying agents. Such modifying agents are well known in the art and are generally ethers, tertiary amines, chelating ethers or chelating amines. Tetrahydrofuran, tetramethylethylene diamine (TMEDA) and diethyl ether are some representative examples of modifying agents which are commonly utilized.
U.S. Pat. No. 5,284,927 discloses a process for preparing a rubbery terpolymer of styrene, isoprene and butadiene having multiple glass transition temperatures and having an excellent combination of properties for use in making tire treads which comprises terpolymerizing styrene, isoprene and 1,3-butadiene in an organic solvent at a temperature of no more than about 40xc2x0 C. in the presence of (a) a tripiperidino phosphine oxide, (b) an alkali metal alkoxide and (c) an organolithium compound.
U.S. Pat. No. 5,534,592 discloses a process for preparing high vinyl polybutadiene rubber which comprises polymerizing 1,3-butadiene monomer with a lithium initiator at a temperature which is within the range of about 5xc2x0 C. to about 100xc2x0 C. in the presence of a sodium alkoxide and a polar modifier, wherein the molar ratio of the sodium alkoxide to the polar modifier is within the range of about 0.1:1 to about 10:1; and wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.01:1 to about 20:1.
U.S. Pat. No. 5,100,965 discloses a process for synthesizing a high trans polymer which comprises adding (a) at least one organolithium initiator, (b) an organoaluminum compound, (c) a group IIa metal alkoxide and (d) a lithium alkoxide to a polymerization medium which is comprised of an organic solvent and at least one conjugated diene monomer.
U.S. Pat. No. 5,100,965 further discloses that high trans polymers can be utilized to improve the characteristics of tire tread rubber compounds. By utilizing high trans polymers in tire tread rubber compounds, tires having improved wear characteristics, tear resistance and low temperature performance can be made. Such high trans polymers include, trans-1,4-polybutadiene, trans styrene-isoprene-butadiene terpolymers, isoprene-butadiene copolymers and trans-styrene-butadiene copolymers.
U.S. Pat. No. 6,103,842 discloses a catalyst system for synthesizing a highly random styrene-butadiene rubber having a high trans content by solution polymerization. The styrene-butadiene rubber made by the process of U.S. Pat. No. 6,103,842 can be utilized in tire tread rubbers that exhibit improved wear characteristics. The catalyst system disclosed by U.S. Pat. No. 6,103,842 consists essentially of (a) an organolithium compound, (b) a group IIa metal alkoxide and (c) a lithium alkoxide. U.S. Pat. No. 6,103,842 further discloses a process for synthesizing a random styrene-butadiene rubber having a low vinyl content by a process which comprises copolymerizing styrene and 1,3-butadiene under isothermal conditions in an organic solvent in the presence of a catalyst system which consists essentially of (a) an organolithium compound, (b) a group IIa metal alkoxide and (c) a lithium alkoxide.
U.S. Pat. No. 4,996,273 discloses a highly active anionic polymerization catalyst containing an organolithium compound, a barium, strontium or calcium compound, and a trialkylaluminum compound containing at least 13 carbon atoms per molecule. These catalyst systems are reported to produce butadiene polymers having a high 1,4-trans-dienyl content.
This invention is based upon the unexpected discovery that a group IIa metal containing catalyst system that is comprised of a) an organolithium compound, (b) a group IIa metal salt selected from the group consisting of group IIa metal salts of amino alcohols and group IIa metal salts of glycol ethers, and (c) an organoaluminum compound, will catalyze the polymerization of conjugated diolefin monomers, such as 1,3-butadiene and isoprene, into rubbery polymers having a high trans microstructure content. The group IIa metal containing catalyst systems of this invention can also be used to copolymerize one or more conjugated diolefin monomers with vinyl aromatic monomers into copolymer rubbers, such as styrene-butadiene rubber. High trans-1,4-polybutadiene rubber and styrene-butadiene rubber that is synthesized using the catalyst system of this invention is highly useful in the preparation of tire tread rubber compounds which exhibit improved wear and tear characteristics, such as tread compounds that contain high levels of silica.
The subject invention more specifically discloses a catalyst system which is comprised of (a) an organolithium compound, (b) a group IIa metal salt selected from the group consisting of group IIa metal salts of amino glycols and group IIa metal salts of glycol ethers, and (c) an organometallic compound selected from the group consisting of organoaluminum compound that contain less than 13 carbon atoms and organomagnesium compounds.
The subject invention also reveals process for synthesizing rubbery polymers having a high trans microstructure by a process that comprises polymerizing a conjugated diolefin monomer in an organic solvent in the presence of a catalyst system that is comprised of (a) an organolithium compound, (b) a group IIa metal salt selected from the group consisting of group IIa metal salts of amino glycols and group IIa metal salts of glycol ethers, and (c) an organometallic compound selected from the group consisting of organoaluminum compounds and organomagnesium compounds.
The present invention further discloses a catalyst system that is comprised of (a) an organolithium compound, (b) a group IIa metal salt selected from the group consisting of group IIa metal salts of amino glycols and group IIa metal salts of glycol ethers, wherein the group IIa metal is selected from the group consisting of beryllium and magnesium, and (c) an organometallic compound selected from the group consisting of organoaluminum compounds and organomagnesium compounds.
The present invention also discloses a catalyst system that is comprised of (a) an organolithium compound, (b) a group IIa metal salt of an amino glycols, and (c) an organometallic compound selected from the group consisting of organoaluminum compounds and organomagnesium compounds.
The subject invention further discloses a catalyst system that is comprised of (a) an organolithium compound, (b) a group IIa metal salt selected from the group consisting of group IIa metal salts of amino glycols and group IIa metal salts of glycol ethers, wherein the group IIa metal is selected from the group consisting of beryllium and magnesium, and (c) an organometallic compound selected from the group consisting of organoaluminum compounds and organomagnesium compounds.
The present invention also reveals a catalyst system that is comprised of (a) an organolithium compound, (b) a group IIa metal salt of an amino glycol, and (c) an organometallic compound selected from the group consisting of organoaluminum compounds and organomagnesium compounds.
The subject invention further discloses catalyst system that is comprised of (a) an organolithium compound, (b) a group IIa metal of N,N-dialkyl amino alkylethoxy ethanol, and (c) an organoaluminum compounds.
The present invention further reveals a catalyst system that is comprised of (a) an organolithium compound, (b) a group IIa metal salt of member selected from the group consisting of tri(ethylene glycol) alkyl ethers and tetra(ethylene glycol) alkyl ethers, (c) an organoaluminum compound.
The subject invention also discloses a process for preparing a group IIa metal salt of an alkyl glycol ether that comprises reacting a group IIa metal hydroxide with the alkyl glycol ether at a temperature which is within the range of about 100xc2x0 C. to about 200xc2x0 C. in the presence of an aromatic organic solvent having a boiling point which is within the range of about 80xc2x0 C. to about 280xc2x0 C. For instance, barium hydroxide can be utilized as the group IIa metal hydroxide. The aromatic organic solvents that can be used include ethyl benzene and mesitylene. The reaction will preferable be conducted at a temperature that is within the range of about 120xc2x0 C. to 180xc2x0 C. and will most preferable be carried out at a temperature which is within the range of about 130xc2x0 C. to 160xc2x0 C.
The polymerizations of the present invention will normally be carried out in a hydrocarbon solvent that can be one or more aromatic, paraffinic or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquid under the conditions of the polymerization. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, methylcyclohexane, isohexane, n-heptane, n-octane, n-hexane, benzene, toluene, xylene, ethylbenzene, diethylbenzene, isobutylbenzene, petroleum ether, kerosene, petroleum spirits, petroleum naphtha, and the like, alone or in admixture.
In the solution polymerizations of this invention, there will normally be from 5 to 30 weight percent monomers in the polymerization medium. Such polymerization media are, of course, comprised of the organic solvent and monomers. In most cases, it will be preferred for the polymerization medium to contain from 10 to 25 weight percent monomers. It is generally more preferred for the polymerization medium to contain 15 to 20 weight percent monomers.
The solution styrene-butadiene rubbers made utilizing the catalyst system and technique of this invention are comprised of repeat units that are derived from the conjugated diolefin monomers and optionally vinyl aromatic monomers, such as styrene. The styrene-butadiene rubbers made utilizing the catalyst system of this invention will typically contain from about 2 weight percent to about 50 weight percent styrene and from about 50 weight percent to about 98 weight percent 1,3-butadiene. However, in some cases, the amount of styrene included will be as low as about 1 weight percent. The styrene-butadiene rubber will more typically contain from about 3 weight percent to about 30 weight percent styrene and from about 70 weight percent to about 97 weight percent 1,3-butadiene. The styrene-butadiene rubber will preferably contain from about 3 weight percent to about 25 weight percent styrene and from about 75 weight percent to about 97 weight percent 1,3-butadiene.
Styrene-butadiene copolymer resins containing from about 50 weight percent to about 95 weight percent styrene and from about 5 weight percent to about 50 weight percent 1,3-butadiene can also be synthesized utilizing the catalyst systems of this invention. Such copolymers having glass transition temperatures within the range of 7xc2x0 C. to 70xc2x0 C. can be used as toner resins.
In the styrene-butadiene rubbers of this invention containing less than about 30 weight percent bound styrene, the distribution of repeat units derived from styrene and butadiene is essentially random. The term xe2x80x9crandomxe2x80x9d as used herein means that less than 10 percent of the total quantity of repeat units derived from styrene are in blocks containing more than five styrene repeat units. In other words, more than 90 percent of the repeat units derived from styrene are in blocks-containing five or fewer repeat units. About 20% of the repeat units derived from styrene will be in blocks containing only one styrene repeat unit. Such blocks containing one styrene repeat unit are bound on both sides by repeat units which are derived from 1,3-butadiene.
In styrene-butadiene rubbers containing less than about 20 weight percent bound styrene which are made with the catalyst system of this invention, less than 4 percent of the total quantity of repeat units derived from styrene are in blocks containing five or more styrene repeat units. In other words, more than 96 percent of the repeat units derived from styrene are in blocks containing less than five repeat units. In such styrene-butadiene rubbers, over 25 percent of repeat units derived from styrene will be in blocks containing only one styrene repeat unit, over 60 percent of the repeat units derived from styrene will be in blocks containing less than 3 repeat units and over 90 percent of the repeat units derived from styrene will be in blocks containing 4 or fewer repeat units.
In styrene-butadiene rubbers containing less than about 10 weight percent bound styrene which are made with the catalyst system of this invention, less than 1 percent of the total quantity of repeat units derived from styrene are in blocks containing 5 or more styrene repeat units. In other words, more than 99 percent of the repeat units derived from styrene are in blocks containing 4 or less repeat units. In such styrene-butadiene rubbers, at least about 50 percent of repeat units derived from styrene will be in blocks containing only one styrene repeat unit and over about 85 percent of the repeat units derived from styrene will be in blocks containing less than 3 repeat units.
The styrene-butadiene copolymers of this invention also have a consistent composition throughout their polymer chains. In other words, the styrene content of the polymer will be the same from the beginning to the end of the polymer chain. No segments of at least 100 repeat units within the polymer will have a styrene content which differs from the total styrene content of the polymer by more than 10 percent. Such styrene-butadiene copolymers will typically contain no segments having a length of at least 100 repeat units which have a styrene content which differs from the total styrene content of the polymer by more than about 5 percent. Additionally, the styrene-butadiene copolymers of this invention having bound styrene contents of up to at least about 42 percent are soluble in mixed hexane solvents.
The polymerizations of this invention are initiated by adding the group IIa metal containing catalyst system to a polymerization medium containing the monomers to be polymerized. Such polymerization can be carried out utilizing batch, semi-continuous or continuous techniques.
The organolithium compounds that can be employed in the process of this invention include the monofunctional and multifunctional initiator types known for polymerizing the conjugated diolefin monomers. The multifunctional organolithium initiators can be either specific organolithium compounds or can be multifunctional types which are not necessarily specific compounds but rather represent reproducible compositions of regulable functionality. The organolithium initiator can also be a functionalized compound.
The choice of initiator can be governed by the degree of branching and the degree of elasticity desired for the polymer, the nature of the feedstock, and the like. With regard to the feedstock employed as the source of conjugated diene, for example, the multifunctional initiator types generally are preferred when a low concentration diene stream is at least a portion of the feedstock, since some components present in the unpurified low concentration diene stream may tend to react with carbon lithium bonds to deactivate the activity of the organolithium compound, thus necessitating the presence of sufficient lithium functionality so as to override such effects.
The multifunctional organolithium compounds which can be used include those prepared by reacting an organomonolithium compounded with a multivinylphosphine or with a multivinylsilane, such a reaction preferably being conducted in an inert diluent such as a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound. The reaction between the multivinylsilane or multivinylphosphine and the organomonolithium compound can result in a precipitate which can be solubilized, if desired, by adding a solubilizing monomer such as a conjugated diene or monovinyl aromatic compound, after reaction of the primary components. Alternatively, the reaction can be conducted in the presence of a minor amount of the solubilizing monomer. The relative amounts of the organomonolithium compound and the multivinylsilane or the multivinylphosphine preferably should be in the range of about 0.33 to 4 moles of organomonolithium compound per mole of vinyl groups present in the multivinylsilane or multivinylphosphine employed. It should be noted that such multifunctional initiators are commonly used as mixtures of compounds rather than as specific individual compounds.
Exemplary organomonolithium compounds include ethyl lithium, isopropyl lithium, n-butyllithium, sec-butyllithium, n-heptyllithium, tert-octyl lithium, n-eicosyl lithium, phenyl lithium, 2-naphthyllithium, 4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium, cyclohexyl lithium, and the like.
Exemplary multivinylsilane compounds include tetravinylsilane, methyltrivinyl silane, diethyldivinylsilane, di-n-dodecyldivinylsilane, cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane, (3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane, and the like.
Exemplary multivinylphosphine compounds include trivinylphosphine, methyldivinylphosphine, dodecyldivinylphosphine, phenyldivinylphosphine, cyclooctyldivinylphosphine, and the like.
Other multifunctional polymerization initiators can be prepared by utilizing an organomonolithium compound, further together with a multivinylaromatic compound and either a conjugated diene or monovinylaromatic compound or both. These ingredients can be charged initially, usually in the presence of a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound as a diluent. Alternatively, a multifunctional polymerization initiator can be prepared in a two-step process by reacting the organomonolithium compound with a conjugated diene or monovinyl aromatic compound additive and then adding the multivinyl aromatic compound. Any of the conjugated dienes or monovinyl aromatic compounds described can be employed. The ratio of conjugated diene or monovinyl aromatic compound additive employed preferably should be in the range of about 2 to 15 moles of polymerizable compound per mole of organolithium compound. The amount of multivinylaromatic compound employed preferably should be in the range of about 0.05 to 2 moles per mole of organomonolithium compound.
Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene, 1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene, 1,3, 5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4xe2x80x2-trivinylbiphenyl, m-diisopropenyl benzene, p-diisopropenyl benzene, 1,3-divinyl-4,5,8-tributylnaphthalene, and the like. Divinyl aromatic hydrocarbons containing up to 18 carbon atoms per molecule are preferred, particularly divinylbenzene as either the ortho, meta or para isomer, and commercial divinylbenzene, which is a mixture of the three isomers, and other compounds such as the ethyl styrenes, also is quite satisfactory.
Other types of multifunctional lithium compounds can be employed such as those prepared by contacting a sec- or tert-organomonolithium compound with 1,3-butadiene, at a ratio of about 2 to 4 moles of the organomonolithium compound per mole of the 1,3-butadiene, in the absence of added polar material in this instance, with the contacting preferably being conducted in an inert hydrocarbon diluent, though contacting without the diluent can be employed if desired.
Alternatively, specific organolithium compounds can be employed as initiators, if desired, in the preparation of polymers in accordance with the present invention. These can be represented by R(Li)x wherein R represents a hydrocarbyl radical containing from 1 to 20 carbon atoms, and wherein x is an integer of 1 to 4. Exemplary organolithium compounds are methyl lithium, isopropyl lithium, n-butyllithium, sec-butyllithium, hexyllithium, tert-octyl lithium, n-decyl lithium, phenyl lithium, 1-naphthyllithium, 4-butylphenyllithium, p-tolyl lithium, 4-phenylbutyllithium, cyclohexyl lithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium, dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane, 1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane, 1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane, 1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene, 1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane, 1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane, 1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane, 1,2,4,6-tetralithiocyclohexane, 4,4xe2x80x2-dilithiobiphenyl, and the like.
The organolithiun compound can be an alkylsilyloxy protected functional lithium compound as described in U.S. Provisional Application Serial No. 60/234,686. The teachings of U.S. Provisional Application Serial No. 60/234,686 are incorporated herein by reference. For instance, the initiator can be an alkylsilyloxy protected functional lithium initiator of the structural formula: (a): 
wherein X represents a group IVa element selected from the group consisting of carbon, germanium, and tin, wherein n represents an integer from 1 to 10, wherein R represents alkyl groups that can be the same or different, wherein the akyl groups contain from 1 to about 8 carbon atoms, and wherein A represents an akylene group; or (b): 
wherein X represents a group IVa element selected from the group consisting of carbon, germanium, and tin, wherein Y represents phosphorous or nitrogen, wherein n represents an integer from 1 to 10, wherein R represents alkyl groups that can be the same or different, wherein the alkyl groups contain from 1 to about 8 carbon atoms, and wherein A represents an alkylene group; or (c): 
wherein R represents alkyl groups that can be the same or different, and wherein the alkyl groups contain from 1 to about 8 carbon atoms, and wherein A represents an alkylene group. The alkylene group can be straight chained or branched. For instance, A can represent a straight chained alkylene group of the structural formula xe2x80x94(CH2)nxe2x80x94 or it can represent a branched alkylene group, such as: 
wherein R represents alkyl groups that can be the same or different, and wherein the alkyl groups contain from 1 to about 8 carbon atoms. R will typically represent an alkyl group containing from 1 to about 4 carbon atoms. It is preferred for R to represent methyl groups.
The alkylsilyloxy protected functional lithium initiator used in the practice of this invention will typically be of the structural formula: 
wherein X represents a group IVa element selected from the group consisting of carbon, silicon, germanium, and tin, wherein n represents an integer from 1 to 10, wherein R represents alkyl groups that can be the same or different, and wherein the alkyl groups contain from 1 to about 8 carbon atoms, or an alkylsilyloxy protected functional lithium compound of the structural formula: 
wherein X represents a group IVa element selected from the group consisting of carbon, silicon, germanium, and tin, wherein Y represents phosphorous or nitrogen, wherein n represents an integer from 1 to 10, wherein R represents alkyl groups that can be the same or different, and wherein the alkyl groups contain from 1 to about 8 carbon atoms. These rubbery polymers will accordingly normally contain a xe2x80x9clivingxe2x80x9d lithium chain end.
It is normally preferred for the alkylsilyloxy protected functional lithium initiator to be of the structural formula: 
wherein n represents an integer from 1 to 10, wherein R represents alkyl groups that can be the same or different, and wherein the alkyl groups contain from 1 to about 8 carbon atoms.
The group IIa metal salts used in the catalyst systems of this invention are selected from the group consisting of group IIa metal salts of amino glycols and group IIa metal salts of glycol ethers. The group IIa metal salts of amino glycols that can be used are typically of the structural formula: 
wherein the R groups can be the same or different and represent alkyl groups (including cycloalkyl groups), aryl groups, alkaryl groups or arylalkyl groups; wherein M represents a group IIa metal selected from the group consisting of beryllium, magnesium, calcium, strontium, and barium; wherein n represents an integer from 2 to about 10; and wherein A represents an alkylene group that contains from about 1 to about 6 carbon atoms. In cases where R represents an alkyl group, the alkyl group will typically contain from 1 to about 12 carbon atoms. In cases where R represents an aryl group, an alkaryl group, or arylalkyl group, the aryl group, alkaryl group, or arylalkyl group will typically contain from about 6 to about 12 carbon atoms. It is typically preferred for R to represent an alkyl group that contains from about 1 to about 8 carbon atoms or a cycloalkyl group that contains from about 4 to about 8 carbon atoms. It is normally more preferred for R to represent an alkyl group that contains from about 1 to about 4 carbon atoms. It is typically preferred for n to represent an integer from about 2 to about 4. It is typically preferred for A to represent an alkylene group that contains from 2 to about 4 carbon atoms with ethylene groups being the most preferred. It is preferred for M to represent strontium or barium with barium being the most preferred.
In cases where R represents cycloalkyl groups the group IIa metal salt will be of the structural formula: 
wherein m represents an integer from 4 to about 8; wherein n represents an integer from 2 to about 10; wherein M represents a group IIa metal selected from the group consisting of beryllium, magnesium, calcium, strontium, and barium; wherein A represents an alkylene group that contains from about 1 to about 6 carbon atoms, and wherein the A groups can be the same or different. It is normally preferred for m to represent an integer from 5 to about 7, for n to represent an integer from about 2 to about 4, and for A to represent an alkylene group that contains from 2 to about 4 carbon atoms. It is preferred for A to represent ethylene groups. It is preferred for M to represent strontium or barium with barium being the most preferred.
Some representative examples of barium salts where R represents cycloalkyl groups include: 
and 
and 
wherein A represents ethylene groups, wherein the A groups can be the same or different, and wherein n represents the integer 2.
The barium salt can also contain a cycloalkyl group that contains an oxygen atom. For instance the barium salt can be of the structural formula: 
wherein A represents ethylene groups, wherein the A groups can be the same or different, and wherein n represents the integer 2.
The group IIa metal salt of glycol ethers that can be used are typically of the structural formula:
Mxe2x80x94((Oxe2x80x94(CH2)n)mxe2x80x94Oxe2x80x94(CH2)xxe2x80x94CH3)2
wherein M represents a group IIa metal selected from the group consisting of beryllium, magnesium, calcium, strontium, and barium; wherein n represents an integer from 2 to 10; wherein m represents an integer from 1 to 6; and wherein x represents an integer from 1 to 12. In is preferred for n to represent an integer from 2 to about 4, for m to represent an integer from 2 to 8, and for x to represent an integer from 1 to 8. It is more preferred for n to represent an integer from 2 to 3, for m to represent an integer from 2 to 4, and for x to represent an integer from 1 to 4. It is preferred for M to represent strontium or barium with barium being the most preferred.
A highly preferred groups IIa metal salt is the barium salt of di(ethyleneglycol)ethyl ether which is of the structural formula:
Baxe2x80x94(Oxe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH2xe2x80x94CH3)2
Another highly preferred group IIa metal salt is 
Other highly preferred group IIa metal salts include barium salts of tri(ethyleneglycol)ethyl ethers and barium salts of tetra(ethyleneglycol)ethyl ethers.
The organoaluminum compounds that can be used in the catalyst systems of this invention are typically of the structural formula: 
in which R1 is selected from the group consisting of alkyl groups (including cycloalkyl), aryl groups, alkaryl groups, arylalkyl groups and hydrogen; R2 and R3 being selected from the group consisting of alkyl groups (including cycloalkyl), aryl groups, alkaryl groups and arylalkyl groups. R1, R2, and R3 will typically represent alkyl groups that contain from 1 to 8 carbon atoms. Some representative examples of organoaluminum compounds that can be utilized are diethyl aluminum hydride, di-n-propyl aluminum hydride, di-n-butyl aluminum hydride, diisobutyl aluminum hydride, diphenyl aluminum hydride, di-p-tolyl aluminum hydride, dibenzyl aluminum hydride, phenyl ethyl aluminum hydride, phenyl-n-propyl aluminum hydride, p-tolyl ethyl aluminum hydride, p-tolyl n-propyl aluminum hydride, p-tolyl isopropyl aluminum hydride, benzyl ethyl aluminum hydride, benzyl n-propyl aluminum hydride and benzyl isopropyl aluminum hydride, trimethyl aluminum, triethyl aluminum, tri-n-propyl aluminum, triisopropyl aluminum, tri-n-butyl aluminum, triisobutyl aluminum, tripentyl aluminum, trihexyl aluminum, tricyclohexyl aluminum, trioctyl aluminum, triphenyl aluminum, tri-p-tolyl aluminum, tribenzyl aluminum, ethyl diphenyl aluminum, ethyl di-p-tolyl aluminum, ethyl dibenzyl aluminum, diethyl phenyl aluminum, diethyl p-tolyl aluminum, diethyl benzyl aluminum and other triorganoaluminum compounds. The preferred organoaluminum compounds include tridodecylaluminum, tri-n-octylaluminum, tri-n-decylaluminum, triethyl aluminum (TEAL), tri-n-propyl aluminum, triisobutyl aluminum (TIBAL), trihexyl aluminum and diisobutyl aluminum hydride (DIBA-H).
The organoaluminum compound will preferably contain less than 13 carbon atoms. Such organoaluninum compounds include trimethylaluminum, triethylaluminum, tri-n-propylaluminum, tri-iso-propylaluminum, tri-isbutylaluminum, tri-t-butylaluminum, and tri-n-butylaluminum.
The molar ratio of the organoaluminum compound to the group IIa metal salt will typically be within the range of about 0.1:1 to about 20:1 and will preferably be within the range of 0.5:1 to 15:1. The molar ratio of the organoaluminum compound to the group IIa metal salt will more preferably be within the range of about 1:1 to about 8:1 and will most preferable be within the range of about 2:1 to about 6:1.
The molar ratio of the organolithium compound to the group IIa metal salt will typically be within the range of about 0.1:1 to about 20:1 and will preferably be within the range of 0.5:1 to 15:1. The molar ratio of the organolithium compound to the group IIa metal salt will more preferably be within the range of about 1:1 to about 6:1 and will most preferable be within the range of about 2:1 to about 4:1.
The organolithium compound will normally be present in the polymerization medium in an amount which is within the range of about 0.01 to 1 phm (parts by 100 parts by weight of monomer). In most cases, from 0.01 phm to 0.1 phm of the organolithium compound will be utilized with it being preferred to utilize from 0.025 phm to 0.07 phm of the organolithium compound in the polymerization medium.
The polymerization temperature utilized can vary over a broad temperature range of from about 20xc2x0 C. to about 180xc2x0 C. In most cases, a temperature within the range of about 40xc2x0 C. to about 120xc2x0 C. will be utilized. It is typically most preferred for the polymerization temperature to be within the range of about 70xc2x0 C. to about 100xc2x0 C. The pressure used will normally be sufficient to maintain a substantially liquid phase under the conditions of the polymerization reaction.
The polymerization is conducted for a length of time sufficient to permit substantially complete polymerization of monomers. In other words, the polymerization is normally carried out until high conversions are attained. The polymerization can then be terminated using a standard technique. The polymerization can be terminated with a conventional noncoupling type of terminator, such as water, an acid, a lower alcohol, and the like, or with a coupling agent.
Coupling agents can be used in order to improve the cold flow characteristics of the rubber and rolling resistance of tires made therefrom. It also leads to better processability and other beneficial properties. A wide variety of compounds suitable for such purposes can be employed. Some representative examples of suitable coupling agents include: multivinylaromatic compounds, multiepoxides; multiisocyanates, multiimines, multialdehydes, multiketones, multihalides, multianhydrides, multiesters which are the esters of polyalcohols with monocarboxylic acids, and the diesters which are esters of monohydric alcohols with dicarboxylic acids, and the like.
Examples of suitable multivinylaromatic compounds include divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene, 1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, and the like. The divinylaromatic hydrocarbons are preferred, particularly divinylbenzene in either its ortho, meta or para isomer. Commercial divinylbenzene which is a mixture of the three isomers and other compounds is quite satisfactory.
While any multiepoxide can be used, liquids are preferred since they are more readily handled and form a relatively small nucleus for the radial polymer. Especially preferred among the multiepoxides are the epoxidized hydrocarbon polymers such as epoxidized liquid polybutadienes and the epoxidized vegetable oils such as epoxidized soybean oil and epoxidized linseed oil. Other epoxy compounds such as 1,2,5,6,9,10-triepoxydecane, and the like, also can be used.
Examples of suitable multiisocyanates include benzene-1,2,4-triisocyanate, naphthalene-1,2,5,7-tetraisocyanate, and the like. Especially suitable is a commercially available product known as PAPI-1, a polyarylpolyisocyanate having an average of three isocyanate groups per molecule and an average molecular weight of about 380. Such a compound can be visualized as a series of isocyanate-substituted benzene rings joined through methylene linkages.
The multiimines, which are also known as multiaziridinyl compounds, preferably are those containing three or more aziridine rings per molecule. Examples of such compounds include the triaziridinyl phosphine oxides or sulfides such as tri(1-ariridinyl)phosphine oxide, tri(2-methyl-1-ariridinyl)phosphine oxide, tri(2-ethyl-3-decyl-1-ariridinyl)phosphine sulfide, and the like.
The multialdehydes are represented by compounds such as 1,4,7-naphthalene tricarboxyaldehyde, 1,7,9-anthracene tricarboxyaldehyde, 1,1,5-pentane tricarboxyaldehyde and similar multialdehyde containing aliphatic and aromatic compounds. The multiketones can be represented by compounds such as 1,4,9,10-anthraceneterone, 2,3-diacetonylcyclohexanone, and the like. Examples of the multianhydrides include pyromellitic dianhydride, styrene-maleic anhydride copolymers, and the like. Examples of the diesters and multiesters include diethyladipate, triethyl citrate, 1,3,5-tricarbethoxybenzene, diethyl phathalate, ethyl benzoate, and the like.
The preferred multihalides are silicon tetrahalides (such as silicon tetrachloride, silicon tetrabromide and silicon tetraiodide) and the trihalosilanes (such as trifluorosilane, trichlorosilane, trichloroethylsilane, tribromobenzylsilane and the like). Also preferred are the multihalogen-substituted hydrocarbons, such as 1,3,5-tri(bromomethyl)benzene, 2,4,6,9-tetrachloro-3,7-decadiene, and the like, in which the halogen is attached to a carbon atom which is alpha to an activating group such as an ether linkage, a carbonyl group or a carbon-to-carbon double bond. Substituents inert with respect to lithium atoms in the terminally reactive polymer can also be present in the active halogen-containing compounds. Alternatively, other suitable reactive groups different from the halogen as described above can be present.
Examples of compounds containing more than one type of functional group include 1,3-dichloro-2-propanone, 2,2-dibromo-3-decanone, 3,5,5-trifluoro-4-octanone, 2,4-dibromo-3-pentanone, 1,2,4,5-diepoxy-3-pentanone, 1,2,4,5-diepoxy-3-hexanone, 1,2,11,12-diepoxy-8-pentadecanone, 1,3,18,19-diepoxy-7,14-eicosanedione, and the like.
In addition to the silicon multihalides as described hereinabove, other metal multihalides, particularly those of tin, lead or germanium, also can be readily employed as coupling and branching agents. Difunctional counterparts of these agents also can be employed, whereby a linear polymer rather than a branched polymer results. Mixed coupling agents containing both silicon multihalides and tin multihalides can also be used.
Broadly, and exemplary, a range of about 0.01 to 4.5 milliequivalents of coupling agent are employed per 100 grams of monomer. It is preferred to utilize about 0.01 to about 1.5 milliequivalents of the coupling agent per 100 grams of monomer to obtain the desired Mooney viscosity. The larger quantities tend to result in production of polymers containing terminally reactive groups or insufficient coupling. One equivalent of treating agent per equivalent of lithium is considered optimum amount for maximum branching, if this result is desired in the production line. The coupling agent can be added in hydrocarbon solution (e.g., in cyclohexane) to the polymerization admixture in the final reactor with suitable mixing for distribution and reaction.
After the copolymerization has been completed, the styrene-butadiene elastomer can be recovered from the organic solvent. The styrene-butadiene rubber can be recovered from the organic solvent and residue by means such as decantation, filtration, centrification and the like. It is often desirable to precipitate the polymer from the organic solvent by the addition of lower alcohols containing from about 1 to about 4 carbon atoms to the polymer solution. Suitable lower alcohols for precipitation of the segmented polymer from the polymer cement include methanol, ethanol, isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. The utilization of lower alcohols to precipitate the rubber from the polymer cement also xe2x80x9ckillsxe2x80x9d the living polymer by inactivating lithium end groups. After the polymer is recovered from the solution, steam-stripping can be employed to reduce the level of volatile organic compounds in the rubber.
There are valuable benefits associated with utilizing the rubbery polymers made with the group IIa metal containing catalyst systems of this invention in tire tread compounds. For instance, styrene-butadiene rubber made with the group IIa metal catalyst system of this invention can be blended with natural rubber to make tread compounds for passenger tires which exhibit outstanding rolling resistance, traction, tear, and tread wear characteristics. In cases where tread wear is of great importance, high cis-1,4-polybutadiene can also be included in the blend. In any case, the styrene-butadiene rubbers of this invention can be used to improve the traction, tread wear and rolling resistance of tires made therewith.